Summary of Investments Other Than Investments In Related Parties (Details) $ in Millions	Dec. 31, 2020 USD ($)
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	$ 15,587
Fair value	0
Balance sheet	21,542
Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	11,312
Fair value	12,338
Balance sheet	12,338
Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3,927
Fair value	8,856
Balance sheet	8,856
Other than Securities Investment
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	348
Fair value	0
Balance sheet	348
States, Municipalities and Political Subdivisions | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	4,604
Fair value	4,997
Balance sheet	4,997
States, Municipalities and Political Subdivisions | Fixed Maturities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	53
Fair value	52
Balance sheet	52
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3,346
Fair value	3,638
Balance sheet	3,638
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	219
Fair value	238
Balance sheet	238
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Indemnity Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	48
Fair value	52
Balance sheet	52
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	357
Fair value	387
Balance sheet	387
States, Municipalities and Political Subdivisions | Fixed Maturities | The Cincinnati Specialty Underwriters Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	580
Fair value	629
Balance sheet	629
States, Municipalities and Political Subdivisions | Fixed Maturities | CSU Producers Resources Inc.
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
United States Government | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	115
Fair value	120
Balance sheet	120
United States Government | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	65
Fair value	70
Balance sheet	70
United States Government | Fixed Maturities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
United States Government | Fixed Maturities | The Cincinnati Indemnity Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
United States Government | Fixed Maturities | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	48
Fair value	48
Balance sheet	48
Foreign Government | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	29
Fair value	29
Balance sheet	29
Foreign Government | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	10
Fair value	10
Balance sheet	10
Foreign Government | Fixed Maturities | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	19
Fair value	19
Balance sheet	19
Government-Sponsored Enterprises | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	12
Fair value	12
Balance sheet	12
Government-Sponsored Enterprises | Fixed Maturities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	11
Fair value	11
Balance sheet	11
Government-Sponsored Enterprises | Fixed Maturities | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
All Other Corporate Bonds | Fixed Maturities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	6,552
Fair value	7,180
Balance sheet	7,180
All Other Corporate Bonds | Fixed Maturities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5
Fair value	5
Balance sheet	5
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	2,964
Fair value	3,211
Balance sheet	3,211
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	103
Fair value	110
Balance sheet	110
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Indemnity Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	30
Fair value	33
Balance sheet	33
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3,192
Fair value	3,540
Balance sheet	3,540
All Other Corporate Bonds | Fixed Maturities | The Cincinnati Specialty Underwriters Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	189
Fair value	211
Balance sheet	211
All Other Corporate Bonds | Fixed Maturities | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	68
Fair value	69
Balance sheet	69
All Other Corporate Bonds | Fixed Maturities | CSU Producers Resources Inc.
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
Common Stock | Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3,640
Fair value	8,541
Balance sheet	8,541
Common Stock | Equity Securities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1,606
Fair value	3,686
Balance sheet	3,686
Common Stock | Equity Securities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1,846
Fair value	4,423
Balance sheet	4,423
Common Stock | Equity Securities | The Cincinnati Casualty Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	56
Fair value	146
Balance sheet	146
Common Stock | Equity Securities | The Cincinnati Indemnity Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	17
Fair value	36
Balance sheet	36
Common Stock | Equity Securities | The Cincinnati Specialty Underwriters Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	96
Fair value	217
Balance sheet	217
Common Stock | Equity Securities | CSU Producers Resources Inc.
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	19
Fair value	33
Balance sheet	33
Nonredeemable Preferred Stock | Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	287
Fair value	315
Balance sheet	315
Nonredeemable Preferred Stock | Equity Securities | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	1
Fair value	1
Balance sheet	1
Nonredeemable Preferred Stock | Equity Securities | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	274
Fair value	299
Balance sheet	299
Nonredeemable Preferred Stock | Equity Securities | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	12
Fair value	15
Balance sheet	15
Policy Loans | Other than Securities Investment | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	33
Fair value	0
Balance sheet	33
Lloyd's and other deposits | Other than Securities Investment | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	109
Fair value	0
Balance sheet	109
Lloyd's and other deposits | Other than Securities Investment | Cincinnati Global
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	53
Fair value	0
Balance sheet	53
Limited Partnership | Other than Securities Investment | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	31
Fair value	0
Balance sheet	31
Limited Partnership | Other than Securities Investment | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	94
Balance sheet	94
Limited Partnership | Other than Securities Investment | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	3
Balance sheet	3
Real Estate | Other than Securities Investment | Cincinnati Financial Corporation
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	5
Balance sheet	5
Real Estate | Other than Securities Investment | The Cincinnati Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	11
Balance sheet	11
Real Estate | Other than Securities Investment | The Cincinnati Life Insurance Company
Summary Of Investments Other Than Investments In Related Parties Reportable Data
Cost or amortized cost	9
Balance sheet	$ 9